A laser irradiation device is used in various optical devices such as laser processing devices and microscopes. Some laser processing devices using said laser irradiation device includes a special light modulator (hereinafter, referred to as an SLM). Patent Literatures 1 to 5 listed below disclose laser processing devices including an SLM.
The laser processing devices described in Patent Literatures 1 and 2 control a laser beam irradiation position in a processing target, and the laser processing device described in Patent Literature 3 controls a laser beam by using an SLM. The laser processing device described in Patent Literature 4 includes a means for measuring wavefront distortion of a laser beam, and corrects measured wavefront distortion by using an SLM. However, this method needs a means for measuring wavefront distortion, and the optical system thereby becomes complicated, and this method is not applicable to the case where measurement of wavefront distortion is impossible such as laser processing.
Patent Literature 5 describes a problem in that aberration occurs when a laser beam is focused on a transparent medium, and a processing point becomes longer in a depth direction, and the laser processing device described in Patent Literature 5 positively utilizes chromatic aberration caused by medium dispersion, etc., and optical path change according to a wavelength on a diffraction element, and controls a processing position by adjusting an intensity of each of light source wavelengths.
Patent Literature 6 describes a method for correcting aberration by applying a phase distribution opposite to known aberration to incident light by a wavefront control element such as an SLM. Here, in Non-Patent Literature 1, spherical aberration caused by inserting a plane parallel plate into an optical system is analytically obtained according to paraxial approximation. Focusing of a laser beam on a transparent medium is equivalent to insertion of a plane parallel plate into an optical system, so that by handling the results described in Non-Patent Literature 1 as known aberration in the method of Patent Literature 6, spherical aberration caused by focusing of a laser beam on a transparent medium can be corrected. However, in this method, the phase range of the phase distribution opposite to the aberration becomes larger over the performance of the wavefront control element, so that the method is not applicable when the laser is irradiated deep inside the medium. Further, an accurate laser irradiation position cannot be obtained.